Method for producing high-carbon steel rails excellent in wear resistance and ductility

ABSTRACT

Disclosed are methods of producing steel rails having a high carbon content and being excellent in wear resistance and ductility from the slabs for rails. One method involves producing a steel rail having a high content of carbon, comprising finish rolling the rail in two consecutive passes, with a reduction rate per pass of a cross-section of the rail of 2-30%, wherein the conditions of the finish rolling satisfy the following relationship: S≦800/(C×T), wherein S is the maximum rolling interval time (seconds), C is the carbon content of the steel, wherein the carbon content is 0.85-1.40 mass %, and T is the maximum surface temperature (° C.) of the rail head. Another method involves producing a steel rail with a high content of carbon, comprising: finish rolling the rail in three or more passes, with a reduction rate per pass of a cross-section of the rail of 2-30%, wherein the conditions of the finish rolling satisfy the following relationship: S≦2400/(C×T×P), wherein S is the maximum rolling interval time (seconds), C is the carbon content of the steel rail, wherein the carbon content is 0.85˜1.40 mass %, T is the maximum surface temperature (° C.) of a rail head, and P is the number of passes, which is 3 or more. In addition to above, controlled additional amounts of V, Nb, N may be added to the steel rail and/or controlled rapid cooling of the rail after rolling may be accomplished to provide further improvements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for producing pearlitic steel railshaving a high content of carbon. The present steel rails are excellentin both wear resistance and ductility and may be used in railroads forcarrying heavy loads.

2. Description of the Related Art

Pearlitic steel having a high carbon content has been used for railroadsdue to its excellent wear resistance. However, the high carbon contenttherein also causes problems of low ductility and toughness. Forexample, steel rails having a typical carbon content (e.g., containingabout 0.6-0.7 mass % of carbon described in JIS (Japan IndustrialStandard) E1101-1990) have a value according to the JIS No. 3 U notchCharpy test at room temperature of around 12-18 J/cm². These steel railshaving such typical carbon content have the problem of brittle fracturescaused by small initial defects and fatigue cracks when used in lowertemperature such as what we call it “cold range”. In recent years, thecarbon content of steel rails has increased in order to improve the wearresistance, which causes further lowering of both the ductility and thetoughness of the steel rails.

Generally, it is said that grain refinement of the pearlite structure,more specifically having a fine-grained structure of both austenite(before transformation to pearlite) and pearlite, is effective insimultaneously improving both ductility and toughness of the steel rail.In order to obtain a fine-grained structure of austenite, a decrease inthe temperature and an increase in the amount of the reduction rate ofthe hot rolling process are carried out. Furthermore, a reheat treatmentat a low temperature after rolling is carried out. In order to obtain afine-grained structure of pearlite, an acceleration of pearlitetransformation from austenite grains by the use of seed transformationis carried out.

However, decreasing the temperature and increasing the amount of thereduction rate of the hot rolling process have a limitation in terms ofmaintaining good formability. This limitation has not allowed productionof a sufficiently fine-grained austenite grain. As for effectingpearlite transformation from austenite grain using seed transformation,it is difficult to control the amount of seed transformation. Thislimitation makes it difficult to perform stable pearlite transformationfrom austenite grain.

In view of above, the following method has been used. This method is onewhere a fine-grained pearlite structure is obtained by pearlitetransformation caused by rapid cooling after reheat treatment of a steelrail at a low temperature following a rolling process. This methodimproves both the ductility and the toughness of the pearlitic steelrail. However, the carbon rails, in which has increased the carboncontent in order to improve the wear resistance, causes a decrease inthe ductility and toughness of the pearlite structure after theaccelerated cooling process. This problem is due to the fact that coarsecarbides remain insoluble in the austenite grains when the reheattreatment at a low temperature is carried out. The reheat process alsointroduces economic problems since it generally increases the productioncost and decreases the productivity.

Research and development of a production method for steel rails having ahigh carbon rails, which simultaneously ensures both formability duringrolling and a fine-grained pearlite structure after rolling has beenrequired. In order to address this requirement, the following productionmethods of high-carbon steel rails have been developed:

-   (1) Japanese Laid-open Patent Hei 07-173530 discloses a production    method for steel rails with high ductility where three or more    consecutive passes of rolling at set intervals of time from one pass    to next pass is carried out in the finish rolling process of high    carbon content steel rails;-   (2) Japanese Laid-open Patent Hei 2001-234238 discloses a production    method for steel rails with a high wear resistance and a high    toughness where two or more consecutive passes of rolling at set    intervals of time from one pass to the next pass is performed, then    continuous rolling and rapid cooling are sequentially carried out in    the finish rolling process of high carbon content steel rails; and-   (3) Japanese Laid-open Patent Hei 2002-226915 discloses a production    method for steel rails with a high wear resistance and a high    toughness where cooling is allowed between passes of rolling    (inter-stand), and continuous rolling and rapid cooling are    sequentially carried out in the finish rolling process of    high-carbon rails.

Features of the rails in the above Japanese Laid-open Patent Hei07-173530, 2001-234238, and 2002-226915 include improved ductility andtoughness of pearlitic steel by obtaining a uniformly sized fine-grainedaustenite grain by continuous rolling thereby achieving a smallreduction. This takes advantage of the fact that steel with high carboncontent is easy to recrystallize at relatively low temperatures and withonly a small reduction.

The continuous rolling methods mentioned above, which are mainlycombinations of the carbon content of steel, the temperature ofcontinuous hot rolling, the number of rolling passes and the timebetween passes, cannot achieve a fine-grain austenite structure. Thisleads to a coarse pearlite structure and results in a failure to improveductility. This is especially true for the method employing coolingbetween passes of rolling (inter-stand), as the rate of grain growthimmediately after rolling is high in high carbon content steel. Thus,the grain growth depends remarkably on the interval of time if coolingis carried out between rollings (inter-stand). Therefore, a fine-grainedaustenite structure is not obtained and the pearlite structure becomescoarse. This results in the problem of no improvement of ductility, evenif the above-mentioned methods of continuous rolling and/or coolingbetween rollings (inter-stand) are applied.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method formanufacturing a rail that is excellent in both ductility and wearresistance by producing a pearlite of fine-grain structure and highhardness.

One embodiment of the invention relates to a method for producing asteel rail having a high content of carbon, comprising: finish rollingthe rail in two consecutive passes, with a reduction rate per pass for across-section of the rail of 2-30%, wherein conditions of the finishrolling satisfy the following relationship: S≦CPT1; wherein the CPT1 isthe value expressed by the following expression 1CPT1=800/(C×T)  (expression 1)wherein S is the maximum rolling interval time (seconds), and (C×T) isdefined as follows; C is the carbon content of the steel, wherein thecarbon content is more than 0.85 mass %, but less than or equal to 1.40mass %, based on the total mass of the steel, and T is the maximumsurface temperature (degree C.) of a rail head. This method produces asteel rail with a high content of carbon that is excellent in wearresistance and ductility.

Another embodiment of the invention relates to a method for producing asteel rail with a high content of carbon, comprising: finish rolling therail in three or more passes, with a reduction rate per pass for across-section of the rail of 2-30%, wherein conditions of the finishrolling satisfy the following relationship: S≦CPT2, wherein the CPT2 isthe value expressed by the following expression 2,CPT2=2400/(C×T×P)  (expression 2)wherein S is the maximum rolling interval time (seconds), and (C×T×P) isdefined as follows; C is the carbon content of the steel rail, whereinthe carbon content is more than 0.85 mass %, but less than or equal to1.40 mass %, based on the total mass of the steel, and T is the maximumsurface temperature (degree C.) of a rail head, and P is the number ofpasses, which is 3 or more. This method produces a steel rail with ahigh content of carbon that is excellent in wear resistance andductility.

In yet another embodiment, the rail of the present invention, inaddition to the carbon, further comprises at least one element in thefollowing list: Si, Mn, Cr, Mo, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, N, V,Nb. The balance of the rail comprises Fe. Additionally, the rail furtheroptionally comprises impurities, which may be unavoidable.

In another embodiment, the chemical composition of the rail meet thefollowing expression: 0.30≧PC≧0.04;

where PC is expressed as the following (expression 3),PC=V (mass %)+10×Nb (mass %)+5×N (mass %)  (expression 3)

In yet another embodiment, the methods of the present invention furthercomprise: immediately after finish rolling, cooling the surface of therail head at a cooling rate of 2-30° C./second until the surfacetemperature reaches 950-750° C. Optionally, after such cooling, when thetemperature of the rail head is more than 700° C., the methods furthercomprise cooling the surface of the rail head at a cooling rate of 2-30°C./second until the surface temperature reaches at least 600° C., andthen allowing the rail to further cool at room temperature (e.g.,approximately 45° F. to 95° F., preferably 65° F. to 85° F.).

In another embodiment, the methods of the present invention furthercomprise: after the finish rolling process, when the temperature of therail head is more than 700° C., cooling the surface of the rail head ata cooling rate of 2-30° C./second until the surface temperature reachesat least 600° C., and then allowing the rail to further cool at roomtemperature (e.g., approximately 45° F. to 95° F., preferably 65° F. to85° F.).

According to the present invention, it is possible with respect to arail to obtain a fine-grained pearlite structure with high-hardness, toimprove the ductility of the rail and to increase the life-span of therail. This is a result of applying one or more of the followingconditions when a high carbon content bloom of rail is continuouslyfinish-rolled to form a rail:

-   (1) The maximum interval time of rolling is controlled to be less    than the time calculated from an expression concerning the carbon    content of steel and the maximum surface temperature of rail at    rolling (rail head) or from an expression concerning the carbon    content of steel and the maximum surface temperature of rail at    rolling (rail head) and the number of passes,-   (2) The additional amounts of V, Nb and N are controlled so as to be    within a range defined based from an expression concerning each    additional amount of V, Nb and N in order to inhibit the growth of    austenite grain caused after the continuous rolling,-   (3) Immediately after the continuous rolling, the surface of the    head of the rail is rapidly cooled down at a predetermined cooling    rate in a predetermined temperature range, and-   (4) Furthermore the surface of the head of the rail in an austenite    phase is rapidly cooled down at a predetermined cooling rate in a    predetermined temperature range to obtain a pearlite structure    excellent in wear resistance and ductility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: Shows the relationship between the maximum temperature of therail head (° C.) and the multiple of S, C, T (S×C×T), where S is themaximum interval time of rolling (seconds), C is the carbon content ofthe steel (mass %), and T is the maximum temperature of rail head (°C.).

FIG. 2: Shows the relationship between the carbon content (mass %) andthe multiple of S, C, T (S×C×T), where S is the maximum interval time ofrolling (seconds), C is the carbon content of the steel (mass %), and Tis the maximum temperature of the rail head (° C.).

FIG. 3: Shows the relationship between the maximum interval time ofrolling (second) and the multiple of S, C, T (S×C×T), where S is themaximum interval time of rolling (seconds), C is the carbon content ofthe steel, and T is the maximum temperature of the rail head (° C.).

FIG. 4: Shows the relationship between the number of rollings (times)and the multiple of S, C, T, P (S×C×T×P), where S is the maximuminterval time of rolling (seconds), C is the carbon content of the steel(mass %), T is the maximum temperature of the rail head (° C.), and P isthe numbers of rollings (times).

FIG. 5: An illustration explaining the different portions of the rail.In FIG. 5, 1 is the top of rail head, and 2 is the head corner.

FIG. 6: Shows the portion of the rail where the specimen for the tensiletest is taken.

FIG. 7: Shows the relationship between the carbon content and the totalelongation value of the rail. In FIG. 7,

• indicates a rail produced by the methods of the invention withoutcontrol of the expression:0.30≧V (mass %)+10×Nb (mass %)+5×N (mass %)≧0.04,□ indicates a rail produced by the methods of the invention with thecontrol of the above expression (PC value, wherein PC=V (mass %)+10×Nb(mass %)+5×N (mass %), therefore, 0.30≧PC≧0.04), andx indicates a rail produced by conventional methods.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present inventors analyzed factors that cause the pearlite structureto be coarse, which is the reason why ductility is not improved. Thisanalysis was performed by studying the combinations of carbon content,the surface temperature at the rail head, the reduction rate of thecross-section of the rail, and the interval time of rolling. Aftervarious experiments, it was found that the grain size of austenitestructure turns coarse after continuous hot rolling if the maximuminterval time during continuous rolling exceeds a certain value.

The present inventors investigated why the grain size of austenitebecome coarse if the maximum interval time of rolling increases. It wasfound that the growth of grains of austenite structure have a positivecorrelation with the carbon content of the steel and the maximum surfacetemperature of the rail head during continuous finish rolling. Inaddition, it was also found that there is a positive correlation betweenthe growth of grains of austenite structure and the number of passes ofrolling, such as when the number of passes of rolling is 3 or more.

Based on the above results, the present inventors carried out ananalysis of multiple correlations on the relationship between theoptimal interval time of rolling for inhibiting the grain size ofaustenite from becoming coarse, the carbon content, the maximum surfacetemperature of the rail head during continuous finish-rolling, and thenumber of rolling passes. The result was that the growth of austenitegrain at the interval of rolling is inhibited and a fine-grainedaustenite structure is obtained if the maximum interval time ofcontinuous rolling is equal to or less than the value calculated byparticular equations. If the number of rolling passes is 2, the equationis one based on the carbon content and the maximum surface temperatureof the rail head. However, if the number of rolling passes is 3 or more,the equation is one based on the carbon content, the maximum surfacetemperature of the rail head, and the number of passes of rolling.

The present inventors also investigated a method for inhibiting thegrowth of austenite grain caused after continuous rolling by controllingprecipitation. It was found that the precipitation of V-carbide,V-Nitride, V-carbonitride, Nb-carbide and Nb-carbonitride generatedduring continuous rolling causes pinning of austenite grains, whichinhibits the growth of austenite grain. In addition, the presentinventors investigated the conditions where the precipitation ofV-carbide, V-Nitride, V-carbonitride, Nb-carbide and Nb-carbonitrideduring the continuous rolling can be fully controlled. The result isthat the growth of austenite grain after continuous rolling is inhibitedenough if the amount of addition of V, Nb and N (mass %) are controlled,respectively, so that the value calculated by an equation based on theamount of addition of V, Nb and N (mass %) can be within a given range.

The present inventors further investigated a method of inhibiting thegrowth of austenite grain after (finishing) continuous rolling byapplying rapid cooling immediately after the rolling. The result is thatthe growth of austenite grain after rolling can be inhibited if thesurface of the rail head is cooled down rapidly within a predeterminedrange of temperature and at a predetermined cooling rate immediatelyafter completing the continuous rolling.

In addition to the above, the present inventors further investigated amethod of obtaining pearlite structure excellent in wear resistance andductility from a fine-grained austenite structure. The result was that apearlite structure having high toughness and fine-grained structure canbe obtained by rapidly cooling the surface of the rail head having anaustenite phase at a predetermined temperature range and at apredetermined cooling rate. The obtained pearlite structure of the railhead retains wear resistance and ductility.

The following are explanations of various limitations defining thepresent invention:

(1) The reason for the limitation on the chemical composition of steelrails: C (carbon) is an element for expediting pearlite transformationand ensuring wear resistance. If the C content is 0.85 mass % or less,it is difficult to ensure a volume ratio of cementite in the pearlitestructure, which makes it difficult to ensure wear resistance in use forrailroads carrying heavy loads. On the other hand, if the C contentexceeds 1.40 mass %, a large amount of pro-eutectoid cementite isgenerated on the old austenite grain boundary, which lowers the wearresistance and the ductility. In view of this, the C content is limitedto the range from more than 0.85 to 1.40 mass %. Preferably, a lowerlimit of C content of 0.95 mass % can highly improve the wearresistance, which greatly improves the life-span of the rail.

With respect to a rail produced using the above-mentioned composition,at least one of Si, Mn, Cr, Mo, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, N, Vand/or Nb can be further added when needed for improving the hardness(strength) of the pearlite structure, for improving ductility of thepearlite structure, for preventing a heat affected zone, for instance awelding zone, from softening, and for controlling the section hardnessdistribution inside the rail head.

The reasons for the limitations by these elements are as follows: Si isan important element as an oxygen scavenger and as an element forincreasing the hardness (strength) of the rail head throughsolid-solution strengthening with a ferrite phase in the pearlitestructure. Besides, Si is an element for inhibiting generation of apro-eutectoid cementite structure in a hyper-eutectoid steel to preventthe lowering of ductility. If the Si content is less than 0.05 mass %,these good effects cannot be significantly expected, and if the Sicontent is more than 2.00 mass %, the weldability is degraded because ofgeneration of oxide and/or generation of a great deal of surface flawsduring hot rolling. In addition, the hardenability is drasticallyincreased and a martensite structure is generated which is detrimentalto the wear resistance and the ductility of the rail. Thus, the Sicontent is limited to the range of from 0.05 to 2.00 mass %.

Mn is an element for increasing the hardenability and for improving thewear resistance by decreasing the pearlite lamellar spacing to ensurethe hardness of the pearlite structure. If the Mn content is less than0.05 mass %, these effects cannot be significantly expected, which makesit difficult to ensure the wear resistance necessary for the rail. Ifthe Mn content is more than 2.00%, the hardenability is drasticallyincreased and a martensite structure is generated which is detrimentalto the wear resistance and the ductility of the rail. Therefore, the Mncontent is limited to the range of from 0.05 to 2.00 mass %.

Cr is an element capable of increasing the equilibrium transformationtemperature, which leads to decrease of the pearlite lamellar spacing toprovide high hardness (strength). Cr is also capable of strengtheningthe cementite phase, which leads to increased hardness (strength) of thepearlite to provide improved wear resistance. If the Cr content is lessthan 0.05 mass %, these effects cannot be significantly expected. If theCr content is more than 2.00%, the hardenability is drasticallyincreased and a martensite structure is largely generated which degradesthe wear resistance and the ductility of the rail. Therefore, the Crcontent is limited to the range of from 0.05 to 2.00 mass %.

Mo is an element capable of increasing the equilibrium transformationtemperature similar to Cr, which leads to decrease of the pearlitelamellar spacing to provide high hardness (strength). If the Mo contentis less than 0.01 mass %, these effects cannot be expected, i.e.improvement of hardness of the rail cannot be significantly expected. Ifthe Mo content is more than 0.50 mass %, the transformation rate isdrastically lowered, which leads to generation of a martensitestructure, which is detrimental to the ductility. In view of this, theMo amount is limited to the range of from 0.01 to 0.50 mass %.

B (boron) is an element for forming iron-carbon boride on the grainboundary of austenite, increasing the fineness of the generatedpro-eutectoid cementite structure, making the pearlite transformationtemperature less dependent on the cooling rate and making hardnessdistribution of the rail head uniform, which prevents degradation ofrail ductility and provides a longer life. If the B content is less than0.0001 mass %, these effects cannot be significantly expected, i.e.there cannot be expected improvement with respect to generation ofpro-eutectoid cementite structure and/or rail head hardness distributionuniformity. If the B content is more than 0.0050 mass %, coarseiron-carbon boride is generated on the austenite grain boundary, whichgreatly lowers the rail ductility and the fatigue-damage resistance.Thus, the B content is limited to the range of from 0.0001 to 0.0050mass %.

Co is an element for making solid-solution with ferrite in the pearlitestructure, which improves the hardness (strength) of the pearlitestructure by solid-solution strengthening. Co is also an element forincreasing the transformation energy of pearlite, which improves theductility by refining the grain of the pearlite structure. Also, Co isan element for improving wear resistance by refining the grain offerrite which is formed by wheel contact on the rail head. If the Cocontent is less than 0.003 mass %, these effects cannot be significantlyexpected. If the Co content is more than 2.00 mass %, the ductility ofthe ferrite phase in the pearlite structure is drastically lowered,which causes spalling damage on the rolling contact surface and lowersthe surface damage resistance of the rail. Therefore, the Co content islimited to the range of from 0.003 to 2.00 mass %.

Cu is an element for making solid-solution with ferrite in the pearlitestructure, which improves the hardness (strength) of the pearlitestructure by solid-solution strengthening. If the Cu content is lessthan 0.01 mass %, these effects cannot be readily expected. If the Cucontent is more than 1.00 mass %, the hardenability is drasticallyincreased and a martensite structure is generated which is detrimentalto the wear resistance of the rail. Further, the ductility of theferrite phase in the pearlite structure is drastically lowered, whichdegrades the ductility of the rail. Therefore, the Cu content is limitedto the range from 0.01 to 1.00 mass %.

Ni is an element for preventing the creation of brittleness during hotrolling caused by adding Cu and for increasing the hardness (strength)of the pearlitic steel by solid-solution strengthening with ferrite. Itis also an element for inhibiting softening in heat-affected zones, forinstance welding zones, by precipitation strengthening (fineprecipitation of Ni₃Ti, intermetallic compound). If the Ni content isless than 0.01 mass %, these effects cannot be readily expected. If theNi content is more than 1.00 mass %, the ductility of the ferrite phaseis drastically lowered, which causes on the rolling contact surface andlowers the surface damage resistance of the rail. Therefore, the Nicontent is limited to the range of from 0.01 to 1.00 mass %.

Ti is a element effective in preventing creation of brittleness of thewelded joint portion by increasing the fineness of the structure of heataffected zones which are heated up to the austenite region takingadvantage of the insolubility of Ti nitride and Ti carbide precipitatedin reheating during welding. If the Ti content is less than 0.0050 mass%, these effects cannot be readily expected. If the Ti content is morethan 0.0500 mass %, coarse Ti nitride and Ti carbide are generated,which greatly lowers the ductility and the fatigue-damage resistance ofthe rail. Thus, the Ti content is limited to the range of from 0.0050 to0.0500 mass %.

Mg is an element effective in improving the ductility of the pearlitestructure by forming fine oxide bonding with O (oxygen), S (sulfur) orAl, inhibiting grain growth of crystal grains during reheating for railrolling and for improving the fineness of the austenite grain. Mg isalso an element effective in improving the ductility of the pearlitestructure by finely dispersing MnS with MgO and/or MgS, forming Mndepleted zones around MnS, expediting the generation of pearlitetransformation, and increasing the fineness of the pearlite block sizeas a result. If the Mg content is less than 0.0005 mass %, these effectscannot be readily expected. If the Mg content is more than 0.0200 mass%, coarse Mg oxide is generated, which greatly lowers the ductility andthe fatigue-damage resistance of the rail. Thus, the Mg content islimited to the range of from 0.0005 to 0.0200 mass %.

Ca is an element effective in improving the ductility of the pearlitestructure by forming sulfide CaS (Ca has a strong bonding force with S),finely dispersing MnS with CaS, forming Mn depleted zone around MnS,expediting the generation of pearlite transformation, and increasing thefineness of the pearlite block size as a result. If the Ca content isless than 0.0005 mass %, these effects cannot be expected. If the Cacontent is more than 0.0150 mass %, coarse Ca oxide is generated, whichlowers the ductility and the fatigue-damage resistance of the rail.Thus, the Ca content is limited to the range of from 0.0005 to 0.0150mass %.

Al is an important element as an oxygen scavenger. Al is also an elementfor shifting the eutectoid transformation temperature toward the side ofa higher temperature and for shifting the amount of eutectoid carbontoward the higher side. Al is also an element effective in inhibitingthe generation of pro-eutectoid cementite structure and in highlystrengthening the pearlite structure. If the Al content is less than0.0100 mass %, these effects cannot be expected. If the Al content ismore than 1.00 mass %, it becomes difficult to dissolve into the steel,which causes generation of coarse aluminum-type inclusions which can bea source of fatigue-damage and lower the ductility and thefatigue-damage resistance of the rail. Also, oxide is formed at welding,which degrades weldability drastically. In view of above, the Al contentis limited to the range of from 0.0100 to 1.00 mass %.

Zr is an element for inhibiting the formation of a segregation zone inthe central region of the billet (bloom, slab) and thereby inhibitinggeneration of pro-eutectoid cementite structures generated in thesegregation region of the rail. This is made by increasing thepercentage of equiaxed crystals (grains) in the solidificationstructure, since ZrO₂ inclusions have a good lattice match, and becomesolidification cores of the high carbon content steel rail of which theprimary crystal is γ-Fe, which enables to form high equi-axed crystalrate in the solidification structure. If the Zr content is less than0.0001 mass %, the number of ZrO₂ inclusions are not enough to work assolidification cores. Consequently a pro-eutectoid cementite structureis generated in the segregation region, which degrades the ductility ofthe rail. If the Zr content is more than 0.2000 mass %, a great amountof coarse Zr type inclusions are generated, which also degrades theductility of the rail and generates fatigue damage resulting from thecoarse Zr type inclusions. This reduces the life-span of the rail.Consequently, the Zr content is limited to the range of from 0.0001 to0.2000 mass %.

N enables the inhibition of grain growth of austenite grain byprecipitating V nitride, V-carbonitride and/or Nb-carbonitride duringcontinuous rolling. N is also an element effective in increasing boththe ductility and the hardness (strength) of the pearlite structure byprecipitating V nitride, V-carbonitride and/or Nb-carbonitride duringthe cooling process after continuous rolling. Further N is an elementeffective in preventing heat affected zones of welded joint parts fromsoftening by precipitating V nitride, V-carbonitride and/orNb-carbonitride in the heat affected zones which is reheated at atemperature range below the Ac1 point. In addition to the above,

N is an element effective in improving the ductility of the pearlitestructure by forming segregation on the austenite grain boundary, whichexpedites pearlite transformation from the austenite grain boundary andincreases the fineness of the pearlite block size. If the N content isless than 0.0060 mass %, the effects mentioned above are very weak. Ifthe N content is more than O.0200 mass %, it becomes difficult todissolve N into the steel to make a solid-solution, which generatesbubbles which can be a source of fatigue damage. In view of this, the Ncontent is limited to the range of from 0.0060 to 0.0200 mass %.Usually, steel rail initially includes N as impurity by a maximum of0.0050 mass %. Consequently, N should be added in amounts sufficient toprovide N in amounts within the range of from 0.0060 to 0.0200 mass % toexpect the above effects.

V enables the inhibition of grain growth of austenite grain byprecipitating V carbide, V nitride, and/or V-carbonitride duringcontinuous rolling. V is also an element effective in increasing boththe ductility and the hardness (strength) of the pearlite structurethrough precipitation-hardening by precipitating V carbide, V nitride,and/or V-carbonitride during the cooling process after continuousrolling. Further V is an element effective in preventing heat affectedzones of welded joint parts from softening by precipitating V carbide, Vnitride, and/or V-carbonitride at relatively a high temperature range inthe heat affected zones, which are reheated at a temperature range belowthe Ac1 point. If the V content is less than 0.005 mass %, these effectscannot be significantly expected, i.e. no significant improvement in theductility and the hardness of the pearlite structure will be achieved.If the V content is more than 0.500 mass %, coarse V carbide, V nitride,and/or V-carbonitride, which can be sources of fatigue-damage, generateand the ductility and the fatigue damage resistance of the rail aredegraded. Thus, the V content is limited to the range of from 0.005 to0.500 mass %.

Nb enables the inhibition of grain growth of an austenite grain byprecipitating Nb carbide, and/or Nb-carbonitride during continuousrolling. Nb is also an element effective in increasing both theductility and the hardness (strength) of the pearlite structure throughprecipitation-hardening by precipitating Nb carbide, and/orNb-carbonitride during the cooling process after continuous rolling.Further, Nb is an element effective in preventing heat affected zones ofwelded joint parts from softening by precipitating Nb carbide, and/orNb-carbonitride at temperatures ranging from low to high in the heataffected zones, which are reheated at a temperature range below the Ac1point. If the Nb content is less than 0.002 mass %, these effects cannotbe significantly expected, i.e. no significant improvement in theductility and the hardness of the pearlite structure can be expected. Ifthe Nb content is more than 0.050 mass %, coarse Nb carbide, and/orNb-carbonitride, which can be sources of fatigue-damage, generate andthe ductility and the fatigue damage resistance of the rail aredegraded. Thus, the Nb content is limited to the range of from 0.002 to0.050 mass %.

(2) The reason for the limitation on the added amount of V, Nb or N,which enables the inhibition of the grain growth of austenite grainafter rolling is as follows. Concerning the above-mentioned V, Nb and N,it is preferable to add these elements in amounts such that (expression3) below is satisfied. The reason why the added amount of V, Nb or N islimited to the range calculated based on the (expression 3) belowconcerning V mass %, Nb mass % and N mass % is now explained. The reasonis that in continuous rolling of high carbon content steel rail, methodsfor inhibiting grain growth of austenite grain after rolling bycontrolling precipitations have been studied. As a result, it was foundthat the precipitation of V-carbide, V-Nitride, V-carbonitride,Nb-carbide and Nb-carbonitride generated during the continuous rollingcauses pinning of austenite grains, which inhibits the growth ofaustenite grain. In addition, the conditions on which the precipitationof V-carbide, V-Nitride, V-carbonitride, Nb-carbide and Nb-carbonitrideduring the continuous rolling can be fully controlled was investigated.It was found that the generation of the precipitation has a positivecorrelation with the added amounts of V, Nb and N.

Based on the above results, the range of added amounts of V, Nb and Nneeded to sufficiently inhibit the growth of austenite grain wasexperimentally investigated. The investigation indicated that thecontribution rate by unit amount (mass %) of V, Nb and N (N is added toexpedite formation of V-nitride, V-, Nb-carbonitride) was different fromeach other. Then, the contribution rate was experimentally obtained andthe (expression 3) below was derived.PC=V (mass %)+10×Nb (mass %)+5×N (mass %)  (expression 3)Using this expression, experiments were carried out with respect tooptimal ranges for amounts of V, Nb and N. As a result it was found thatif the value of PC defined by the (expression 3) is less than 0.04, thegrowth of austenite grain after continuous rolling cannot be inhibitedsince the pinning force with austenite grain was too small; and if thevalue of PC is more than 0.30, the growth of an austenite grain aftercontinuous rolling cannot be significantly inhibited where theproperties of the rail are not adversely affected but coarse V-carbide,Nb-carbide, V-nitride, V-carbonitride, Nb-carbonitride are generated,which degrade the pinning force with austenite. In view of above, the PCvalue is limited as described in the following expression:0.30≧PC≧0.04.

As mentioned above, N is added in order to expedite the formation ofprecipitation V-nitride, V-carbonitride, and Nb-carbonitride. Additionof N alone does not form the above precipitations, i.e. there is noeffect of inhibiting the growth of the austenite grain. Consequently, inorder to inhibit the growth of the austenite grain, addition of V alone,Nb alone, or addition of a combination of V and Nb, V and N, Nb and N,or V, Nb and N can be made. In the case wherein N alone is added, inother words, when neither V nor Nb are added, the value of PC isregarded as 0 (zero) mass %. As mentioned above, steel rail usuallycontains N as an impurity in an amount of about 0.0050 mass % atmaximum. In order to expedite the formation of V-nitride,V-carbonitride, and/or Nb-carbonitride, N is added so that the N contentbecomes equal to or more than 0.0060 mass %. Therefore, in thecalculation of the (expression 3) above, the N content is assumed to be0 (zero) mass % if the N content is less than 0.0060 mass %.

(3) The reason for the limitation of the cross-section reduction rateper pass is as follows. The cross-section reduction rate per pass of therail in the finish rolling is limited to the range of from 2 to 30%. Ifthe cross-section reduction rate of the rail is more than 30%, a greatamount of heat is generated, which largely increases the temperature ofthe rail head surface. This causes the austenite grain of the rail headto become coarse, which makes it difficult to ensure the ductility ofthe rail. In addition, it also becomes difficult to ensure theformability during rail rolling. If the cross-section reduction rate perpass in the finish rolling is less than 2%, it is not possible to obtainthe necessary strain to re-crystallize the austenite grain of the railhead. Therefore, the austenite grain is not fine-grain, which fails toensure the ductility of the rail. Thus, the cross-section reduction rateper pass in the finish rolling is limited to the range from 2 to 30%.

The reason for the limitation of the maximum interval time of rolling isas follows. The maximum interval time of rolling (S in seconds) islimited to a time equal to or less than the value calculated from thefollowing two expressions (expression (1) and expression (2) below). Theexperiment involving two passes of continuous rolling with a 2-30%cross-sectional reduction rate per pass is carried out with respect tohigh carbon content steel rail while changing the conditions of maximumrolling interval time (S), the carbon content of the steel (C, mass %),and the maximum surface temperature of the rail head (T, ° C.) and theductility (total elongation value) of the steel rail was checked by atensile test. As comparison examples, steel containing the same chemicalcomposition is rolled with the conditions of one pass, a rollingtemperature of 950° C. and a cross-section reduction rate of 10%, andthe ductility (total elongation value) was checked in the same manner.

FIG. 1 shows the results of a continuous rolling experiment. Theexperimental conditions were: carbon content (C, mass %) of the steelwas 1.0 mass %, the cross-section reduction rate was 2-30% per pass, themaximum rolling interval time (S, seconds) was 0.8 seconds, the numberof passes was 2, and the maximum surface temperature of the rail headwas changed (T, ° C.). The vertical axis represents (S×C×T) and thehorizontal axis represents the maximum surface temperature of the railhead (T, ° C.).

FIG. 2 shows the result of another continuous rolling experiment. Theexperimental conditions were: the carbon content (C, mass %) of thesteel was changed, the cross-section reduction rate was 2-30% per pass,the maximum rolling interval time (S, seconds) was 0.8 seconds, thenumber of passes was 2, and the maximum surface temperature of the railhead was 950° C. The vertical axis represents (S×C×T) and the horizontalaxis represents the carbon content (C, mass %).

FIG. 3 shows the results of another continuous rolling experiment. Theexperimental conditions were: the carbon content (C, mass %) of thesteel was 1.0 mass %, the cross-section reduction rate was 2-30% perpass, the maximum rolling interval time (S, seconds) was changed, thenumber of passes was 2, and the maximum surface temperature of the railhead was 950° C. The vertical axis represents (S×C×T) and the horizontalaxis represents the maximum rolling interval time (S, seconds).

As shown in FIGS. 1-3, when the value (S×C×T) exceeds 800, improvementof the ductility (total elongation value) becomes insufficient comparedto the comparison examples. This result is independent of the change ofindividual S, C, and T. When the value (S×C×T) exceeds 900, there is nodifference in improvement of the ductility (total elongation value)compared to comparison examples. When the value (S×C×T) becomes smallerthan 800, the ductility is drastically improved compared to comparisonexamples.

Next, the effect of the number of passes in continuous rolling wasevaluated. FIG. 4 shows the results of another continuous rollingexperiment. The experimental conditions were: the carbon content (C,mass %) of the steel was 1.0 mass %, the cross-section reduction ratewas 2-30% per pass, the maximum rolling interval time (S, seconds) was0.5 seconds, the number of passes (P, times) was changed (3-6 passes),and the maximum surface temperature of the rail head (T, ° C.) was 950°C. As comparison examples, steel containing the same chemicalcomposition was rolled with the conditions of one pass, a rollingtemperature of 950° C. and a cross-section reduction rate of 10% perpass, and the ductility (total elongation value) was checked in the samemanner. The vertical axis represents (S×C×T×P) and the horizontal axisrepresents the number of passes (P, times) in the continuous finishrolling.

As shown in FIG. 4, when the value (S×C×T×P) exceeds 2400, improvementof the ductility (total elongation value) becomes insufficient comparedto the comparison examples. When the value (S×C×T×P) exceeds 2600, thereis no difference with improvement of the ductility (total elongationvalue) compared to the comparison examples. When the value (S×C×T×P)becomes smaller than 2400, the ductility is drastically improvedcompared to the comparison examples.

The present inventors have studied the operation conditions ofcontinuous rolling to ensure the ductility (total elongation value)using the correlations described above. In the actual rolling processfor producing a commercial rail, it is difficult to change the carboncontent of the steel (C, mass %) and the number of passes (P, times)since the wear resistance and the rolling formability have to beensured. In view of this, the maximum rolling interval time (S, seconds)and the maximum surface temperature of the rail head (T, ° C.) arecontrolled. As mentioned above, the maximum rolling interval time (S,seconds) and the ductility (total elongation value) have a correlation.As S increases, both (S×C×T) and (S×C×T×P) are increased, and theductility (total elongation value) is lowered. The present inventorscame up with idea that if the maximum rolling interval time S was keptlower than the values shown below for expressions (1) and (2), which aredetermined from the relation above, then the ductility (total elongationvalue) of the steel rail would be improved. As a result of the rollingexperiments for commercial rail, it was found that in order to inhibitthe growth of austenite grain at inter-stand (standing betweenconsecutive passes) and to increase the fineness of austenite grainafter continuous rolling, if the number of passes is 2, the maximumrolling interval time S has to be less than or equal to the value CPT1calculated from the following (expression 1) consisting of C (mass %) ofthe carbon content of the steel and T (° C.) of the maximum surfacetemperature of a rail head during the rolling, and if the number ofpasses is 3 or more, the maximum rolling interval time S has to be lessthan or equal to the value CPT2 calculated from the following(expression 2) consisting of C (mass %) of the carbon content of thesteel, T (° C.) of the maximum surface temperature of a rail head duringthe rolling and P (number of times) of the number of passes.CPT1=800/(C×T)  (expression 1)CPT2=2400/(C×T×P)  (expression 2)S (sec)≧CPT1, CPT2

Definitions:

The rolling interval time means the time that a blank (billet, bloom,slab) needs to travel from one rolling stand (pass) to next rollingstand (pass), wherein each of the rolling stands is required to beoperated with the reduction rate of 2% or more. In other words, if aparticular rolling stand in the continuous finish rolling process isoperated with the reduction rate less than 2%, the particular standcannot be taken into account for determining the rolling interval time,but rather be ignored. The maximum rolling interval time means thelongest time among the rolling interval times. In the case of 3 passes(3 rolling stands), for example, if the time A taken between first passand second pass is longer than the time B taken between second pass andthird pass, then the time A is the Maximum rolling interval time.

The surface temperature of the rail head (T, ° C.) is the surfacetemperature of the rail head measured between each consecutive pass. Themaximum surface temperature of the rail head is the highest temperatureamong those measured.

(5) The reason for the limitation of the condition on the rapid coolingof the rail head immediately after hot rolling is as follows. If thecooling rate for cooling the rail head immediately after hot rolling isless than 2° C./sec., the austenite grains become coarse during thecooling, which degrades the ductility of the rail head. If the coolingrate for cooling the rail head immediately after hot rolling is morethan 30° C./sec., a large amount of heat recuperation from inside therail head generates after the rapid cooling, which raises thetemperature of the surface of the rail head to form coarse austenitegrains and leads to degradation of the ductility. Therefore, the coolingrate for the rail head immediately after hot rolling is limited to therange of 2-30° C./sec.

As for the temperature range within which the rapid cooling is applied,if the rapid cooling is terminated at a temperature of more than 950°C., austenite grains may significantly grow depending on the carboncontent of the steel, which causes coarse grains of austenite anddegrades the ductility of the rail head. If the rapid cooling is stillapplied after the temperature reaches below 750° C., a large amount ofheat returning from inside the rail head may generate depending on therate of cooling, which raises the temperature of the surface of the railhead and generates coarse austenite grains, which lower the ductility.In view of this, the temperature range within which the rapid cooling isapplied is limited to the range of 950-750° C.

(6) The reason for the limitation of the condition on rapid cooling ofthe head of the rails after hot rolling is as follows. This is a finalheat treatment performed after hot rolling. When the temperature of therail head falls below 700° C., pearlite transformation will commence.Therefore, if the rapid cooling on this stage starts after thetemperature of the rail head falls below 700° C., the hardness of therail head cannot be increased and this will fail to improve the wearresistance. Also, depending on the carbon content and/or alloy elements,pro-eutectoid cementite structures are generated, which degrades theductility of the rail head. Therefore, the starting temperature for therapid cooling at the final stage after hot rolling is limited to atemperature higher than 700° C.

As for the range of the rapid cooling rate, if the rapid cooling rate ofthe surface of the rail head is less than 2° C./second, no improvementon hardness of the rail head can be seen. Besides, pro-eutectoidcementite may be generated depending on the carbon content and/or alloyelements, which degrades the ductility. And, if the rapid cooling rateis more than 30° C./second, a martensite structure is generated in thepresent composition system, which significantly degrades the ductilityof the rail head. Thus, the rapid cooling rate is limited to a range of2-30° C./second.

As for the temperature to which the rapid cooling is terminated, if therapid cooling is terminated at a temperature of more than 600° C., alarge amount of heat returning from inside the rail is generated. As aresult, the temperature rise causes pearlite transformation, which leadsto the failure of hardening the pearlite structure, i.e., failure ofensuring wear resistance. This also causes the pearlite structure tobecome coarse, which degrades the ductility of the rail head surface.Therefore, the rapid cooling has to be performed until the temperaturereaches at least 600° C. There is no limitation on the lowertemperature. However, 400° C. is the practical lower limit consideringthe requirements of ensuring the hardness of the rail head surface andpreventing the martensite structure from being formed in the segregationregion inside the rail head.

FIG. 5 shows the names of the parts of a rail. Shown are the top of therail head 1 and the head corner 2. The “surface temperature of the railhead” of the present invention described herein refers to the surfacetemperature at the top of head 1 and the head corners 2, 2 in FIG. 5. Bycontrolling the surface temperature as discussed above, austenite graincan be fine-grained at the rolling and the ductility of the rail isimproved. Likewise, the temperatures relating to the heat treatmentsperformed immediately after or after continuous hot rolling, such as therapid cooling process, refer to the temperature of surface of the top ofhead 1 and the head corners 2,2, or the temperature of the region withina depth of 5 mm from the head surface. By controlling the temperature ofthis region, a fine-grained pearlite structure having an excellent wearresistance can be obtained.

In this producing method, coolant used for cooling is not limited.However, air, mist, and a mixture of air and mist are preferable toensure controlled cooling. The metal structure of the rail head producedby the present invention should preferably be a pearlite structure. Aslight amount of pro-eutectoid ferrite structure, pro-eutectoidcementite structure and bainite structure may generate in the pearlitestructure depending on the selection of chemical composition and/orselection of rapid cooling conditions. However, a slight amount of thesestructures in the pearlite structure do not significantly affect thefatigue strength or the ductility. A rail head produced using thepresent invention can therefore include a slight amount of pro-eutectoidferrite structure, pro-eutectoid cementite structure and bainitestructure.

EXAMPLES

TABLE 1 shows the chemical composition of the tested steel rails.

TABLE 2 shows the elements (carbon content, PC value), hot rollingconditions, heat treatment conditions, micro-structures, hardnesses andtotal elongation values of tensile test of the rails produced by themethods of the invention from the tested steel rails.

TABLE 3 shows the elements (carbon content, PC value), hot rollingconditions, heat treatment conditions, micro-structures, hardnesses andtotal elongation values of tensile test of the rails produced byconventional methods from the tested steel rails.

The rails for the examples are as follows:

-   (1) Rails produced by the methods of the invention (26 rails listed    in TABLE 2 denoted by Nos. 1-26).    -   Rails denoted Nos. 1-4, and 6-15: chemical composition are shown        in TABLE 1 and the hot rolling conditions and heat treatment        conditions are shown in TABLE 2.    -   Rails denoted Nos. 5, and 16-26: chemical composition are shown        in TABLE 1 and the hot rolling conditions, reheat treatment        conditions and PC values are shown in TABLE 2.-   (2) Rails produced by methods for comparison (18 rails listed in    TABLE 3 denoted by Nos. 27-44).    -   Rails denoted 27-44: chemical composition are shown in TABLE 1        and the hot rolling conditions are shown in TABLE 3.        (1) Tensile Test of the Head Rails        Test Machine: all-purpose miniature tensile test machine        Form of the specimen: Similar to JIS No. 4,        Length of parallel part: 25 mm,        Diameter of parallel part: 6 mm,        The distance for measurement and evaluation of elongation: 21        mm,        The portion of rails where specimens were taken: 5 mm below the        head of the rails (See FIG. 6),        The rate of tensile: 10 mm/min,        Temperature: room temperature (approx. 20° C.)

The following are explanations based on the examples.

TABLE 1 shows the chemical composition of steel for tested railexamples.

TABLE 2 shows the conditions of hot rolling, cooling after rolling andheat treatment after rolling and the properties of the heads of therails produced by the method of the present invention.

TABLE 3 shows the conditions of hot rolling, cooling after rolling andheat treatment after rolling and the properties of the heads of therails produced by conventional methods.

First, the effects of the hot rolling conditions (the maximum intervaltime of rolling ≦ values in expressions (1) and (2) above) areexplained. The chemical composition of rail 10 in TABLE 2 and rail 32 inTABLE 3 are the same in composition as steel G in TABLE 1, whose carboncontent is 1.10 mass %. The value of the total elongation on the tensiletest of rail 10 in TABLE 2 is 2.0% higher than that of rail 32 in TABLE3, the former and the latter being 11.7% and 9.7% respectively. Thedifference comes from the fact that the maximum interval time of rollingof rail 10 in TABLE 2 is controlled less than expression (2) value andthis control makes the austenite structure fine-grained.

Second, the effects of the PC value are explained. The chemicalcomposition of rail 10 in TABLE 2 and rail 24 in TABLE 2 are the same incomposition as steel G in TABLE 1, whose carbon content is 1.10 mass %.The value of the total elongation on the tensile test of rail 24 inTABLE 2 is 1.0% higher than that of rail 10 in TABLE 2, the former andthe latter being 12.7% and 11.7%, respectively. The difference comesfrom the fact that the PC value of rail 10 is not within the range of0.04-0.30, while the PC value of rail 24 is within the stated range of0.04-0.30, and this inhibits the growth of the austenite grains afterrolling.

On the other hand, the value of the total elongation on the tensile testof the rail 37 in TABLE 3, whose PC value is not within the range of0.04-0.30, is deteriorated to 11.0%, because the coarse V—Nb carbide, Vnitride, and V—Nb carbonitride are generated if the PC value is notwithin the stated range. As stated above, the control of the maximuminterval time of the rolling must meet expressions (1) or (2) above, andwith continuous rolling this improves the ductility of the rails. Inaddition, control of the PC value within the range of 0.04-0.30 improvesthe ductility of the rails.

FIG. 7 shows the relationship between the carbon content and the valueof the total elongation on the tensile test, which have the mostinfluence on the ductility. As shown in FIG. 7, compared with rails27-36, produced by a conventional method, rails 1-4 and 6-15, producedby the methods of the present invention have improved ductility on thehead of the rails in any amount of carbon content because the maximuminterval time of the rolling is controlled. In particular, rails 5 and16-26 have further improved ductility on the head of rails in any amountof carbon content because not only the maximum interval time of rollingbut also the PC value, which is calculated by the equation based on V,Nb, N, is controlled within the required range of 0.04-0.30 in rails 5and 16-26. Therefore, not only is the austenite structure madefine-grained but also the growth of the austenite grains is inhibited.

Third, the effects of cooling after rolling are explained. The chemicalcomposition of rail 10 in TABLE 2 and rail 38 in TABLE 3 are the same incomposition as steel G in TABLE 1, whose carbon content is 1.10 mass %.The value of the total elongation on the tensile test of rail 10 inTABLE 2 is 2.9% higher than that of rail 38 in TABLE 3, the former andthe latter being 11.7% and 8.8%, respectively. The difference comes fromthe fact that the cooling rate of rail 10 in TABLE 2 is controlledwithin the inventive range and this control inhibits the growth of theaustenite grains, while the cooling rate of rail 38 in TABLE 3 is notcontrolled.

The chemical composition of rail 8 in TABLE 2 and rail 39 in TABLE 3 arethe same as that of steel F in TABLE 1, whose carbon content is 1.00mass %. The value of the total elongation on the tensile test of rail 8in TABLE 2 is 1.7% higher than that of rail 39 in TABLE 3, the formerand the latter being 11.2% and 9.5%, respectively. The difference comesfrom the fact that the temperature of termination of rapid cooling ofrail 8 in TABLE 2 is controlled within the inventive range and thiscontrol inhibits the growth of the austenite grains, while thetemperature of termination of rapid cooling of rail 39 in TABLE 3 is notcontrolled.

The chemical composition of rail 10 in TABLE 2 and rail 40 in TABLE 3are the same in composition as steel G in TABLE 1, whose carbon contentis 1.10 mass %. The value of the total elongation on the tensile test ofrail 10 in TABLE 2 is 3.2% higher than that of rail 40 in TABLE 3, theformer and the latter being 11.7% and 8.5%, respectively. The differencecomes from the fact that the temperature of termination of rapid coolingof rail 10 in TABLE 2 is controlled within the inventive range and thiscontrol inhibits the growth of the austenite grains, while thetemperature of termination of rapid cooling of rail 40 in TABLE 3 is notcontrolled.

Fourth, the effects of heat treatment are explained. The chemicalcomposition of rail 2 in TABLE 2 and rail 41 in TABLE 3 are the same incomposition as steel A in TABLE 1, whose carbon content is 0.86 mass %.The value of the total elongation on the tensile test of rail 2 in TABLE2 is 11.5% higher than that of rail 41 in TABLE 3, the former and thelatter being 16.5% and 5.0%, respectively. The difference comes from thefact that the cooling rate of rail 2 in TABLE 2 is controlled within theinventive range and this control inhibits the generation of martensite,while the cooling rate of rail 41 in TABLE 3 is not controlled.

The chemical composition of rail 11 in TABLE 2 and rail 42 in TABLE 3are the same in composition as steel G in TABLE 1, whose carbon contentis 1.10 mass %. The value of the total elongation on the tensile test ofrail 11 in TABLE 2 is 4.6% higher than that of rail 42 in TABLE 3, theformer and the latter being 11.5% and 6.9%, respectively. The differencecomes from the fact that the cooling rate of rail 11 in TABLE 2 iscontrolled within the inventive range and this control inhibits theprecipitation of martensite, while the cooling rate of rail 42 in TABLE3 is not controlled.

The chemical composition of rail 13 in TABLE 2 and rail 43 in TABLE 3are the same as that of steel H in TABLE 1, whose carbon content is 1.10mass %. The value of the total elongation on the tensile test of rail 13in TABLE 2 is 5.3% higher than that of rail 43 in TABLE 3, the formerand the latter being 11.5% and 6.2%, respectively. The difference comesfrom the fact that the cooling rate of rail 13 in TABLE 2 is controlledwithin the inventive range and this control inhibits the pro-eutectoidcementite, while the cooling rate of rail 43 in TABLE 3 is notcontrolled.

The chemical composition of rail 5 in TABLE 2 and rail 44 in TABLE 3 arethe same in composition as steel D in TABLE 1, whose carbon content is0.95 mass %. The value of the total elongation on the tensile test ofrail 5 in TABLE 2 is 5.6% higher than that of rail 44 in TABLE 3, theformer and the latter being 13.2% and 7.6%, respectively. The differencecomes from the fact that the temperature of termination of cooling ofrail 5 in TABLE 2 is controlled within the inventive range and thiscontrol inhibits the coarseness of the grain of pearlite structure,while the temperature of termination of cooling of rail 44 in TABLE 3 isnot controlled.

As stated above, not only is a fine-grain austenite and pearlitestructure obtained but also the stable generation of a pearlitestructure is obtained by controlling the cooling conditions and the heattreatment conditions after rolling which causes the improvement ofductility (the total elongation value) of the rails.

Compared with the rails 27-44, rails 1-26 have improved wear resistanceand ductility. The mechanisms are as follows:

As for rails 1-26, the maximum interval time of rolling is controlled tobe less than the value calculated by the equation based on the carboncontent of steel, the maximum head temperature of the rails duringcontinuous rolling and the number of rolling passes and rapid cooling iscarried out immediately after rolling. These controls provide afine-grain austenite structure. In addition, as for rails 1-26, the heattreatment process after rolling at a suitable temperature range and at asuitable cooling rate is carried out. These controls inhibit thegeneration of pro-eutectoid cementite and martensite, which aredetrimental to the ductility of the rails.

All cited patents, publications, copending applications, and provisionalapplications referred to in this application are herein incorporated byreference.

This application claims priority to Application Nos. JP-2004-65676 andJP-2004-285934, filed in Japan on Mar. 9, 2004 and Sep. 30, 2004,respectively, the entire contents of which are herein incorporated byreference.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims. TABLE1 Chemical Composition of Steels Chemical Composition (mass %) Steel CSi/Mn/Cr/Mo/V/Nb/B/Co/Cu/Ni/Ti/Mg/Ca/Al/Zr/N A 0.86 Si: 0.25, Mn: 0.65,Cu: 0.25, Co: 0.05, Ni: 0.25 B 0.90 Si: 0.54, Mn: 0.92, Cr: 0.15, N:0.0120 C 0.90 Si: 0.22, Mn: 0.75, Cr: 0.21, Ti: 0.0150, B: 0.0022 D 0.95Si: 0.70, Mn: 0.60, Cr: 0.55, V: 0.03, Nb: 0.015 E 1.00 Si: 0.40, Mn:0.75, Cr: 0.28 F 1.00 Si: 0.75, Mn: 0.45, Cr: 0.55 G 1.10 Si: 0.65, Mn:0.70, Cr: 0.25, Zr: 0.0015 H 1.10 Si: 1.20, Mn: 1.15, Cr: 0.22, Ti:0.0130, Al: 0.07 I 1.20 Si: 0.65, Mn: 0.35, Ca: 0.0025 J 1.40 Si: 0.25,Mn: 0.55, Mg: 0.0020, Mo: 0.03 K 0.95 Si: 0.70, Mn: 0.60, Cr: 0.55, V:0.03, Nb: 0.015, N: 0.0080 L 1.00 Si: 0.40, Mn: 0.75, Cr: 0.28, V: 0.02,N: 0.0060 M 1.00 Si: 0.40, Mn: 0.75, Cr: 0.28, V: 0.05, N: 0.0080 N 1.00Si: 0.40, Mn: 0.75, Cr: 0.28, V: 0.07 O 1.00 Si: 0.75, Mn: 0.45, Cr:0.55, V: 0.25 P 1.10 Si: 0.65, Mn: 0.70, Cr: 0.25, V: 0.07, N: 0.0120 Q1.10 Si: 0.65, Mn: 0.70, Cr: 0.25, Nb: 0.015 R 1.10 Si: 0.65, Mn: 0.70,Cr: 0.25, Zr: 0.0015, Nb: 0.010, N: 0.0080 S 1.10 Si: 0.65, Mn: 0.70,Cr: 0.25, Zr: 0.0015, V: 0.07, Nb: 0.015 T 1.20 Si: 0.65, Mn: 0.35, Ca:0.0025, V: 0.06 U 1.40 Si: 0.25, Mn: 0.55, Mg: 0.0020, Mo: 0.03, V:0.05, Nb: 0.010 V 1.10 Si: 0.65, Mn: 0.70, Cr: 0.25, Zr: 0.0015, V:0.10, Nb: 0.04(the balance Fe and unavoidable impurities)

TABLE 2 Rails produced by the method of the invention Carbon content andPC Value Hot rolling conditions PC Number of Range of cross- Maximum Ccontent value rollings section reduction temperature of (CPT1) MethodNo. Steel (mass %) V + 10 × Nb + 5 × N P(times) (%) rail head (T, deg.C.) 800/C. × T Inventive 1 A 0.86 0.00 2 5˜20 1000 0.93 2 A 0.86 0.00 38˜24 950 — 3 B 0.90 0.00 4 10˜30  950 — 4 C 0.90 0.00 2 10˜28  920 0.975 D 0.95 0.18 4 10˜24  950 — 6 E 1.00 0.00 2 15˜30  950 0.84 7 E 1.000.00 5 8˜18 875 — 8 F 1.00 0.00 5 10˜20  1000 — 9 F 1.00 0.00 6 2˜15 900— 10 G 1.10 0.00 4 4˜25 930 — 11 G 1.10 0.00 2 10˜28  920 0.79 12 H 1.100.00 5 2˜25 900 — 13 H 1.10 0.00 6 5˜10 850 — 14 I 1.20 0.00 5 10˜30 900 — 15 J 1.40 0.00 5 10˜30  940 — 16 K 0.95 0.22 4 10˜24  950 — 17 L1.00 0.05 5 8˜18 875 — 18 M 1.00 0.09 5 8˜18 875 — 19 N 1.00 0.07 5 8˜18875 — 20 O 1.00 0.25 6 2˜15 900 — 21 P 1.10 0.13 4 4˜25 930 — 22 Q 1.100.15 4 4˜25 930 — 23 R 1.10 0.14 4 4˜25 930 — 24 S 1.10 0.22 4 4˜25 930— 25 T 1.20 0.06 5 10˜30  900 — 26 U 1.40 0.16 5 10˜30  940 — Hotrolling conditions Heat treatment condition after rolling Maximuminterval Cooling condition after rolling Temperature of Temperature of(CPT2) time of rolling (S, Cooling rate Temperature of ending startingheat Cooling rate ending cooling Method No. 2400/C. × T × P sec.) (°C./sec.) cooling (° C.) treatment (° C.) (° C./sec.) (° C.) Inventive 1— 0.8 5 900 750 7 580 2 0.98 0.7 6 945 710 2 520 3 0.70 0.6 7 900 890 5510 4 — 0.6 air cooling after rolling 760 8 530 5 0.66 0.5 8 870 740 10480 6 — 0.7 10 850 930 9 500 7 0.55 0.4 air cooling after rolling 760 10480 8 0.48 0.2 11 835 770 6 510 9 0.44 0.3 7 820 730 11 450 10 0.59 0.415 820 800 12 450 11 — 0.6 18 800 750 14 480 12 0.48 0.3 air coolingafter rolling 780 12 465 13 0.43 0.1 20 780 780 15 500 14 0.44 0.3 24780 770 20 550 15 0.36 0.2 28 760 760 25 480 16 0.66 0.5 8 870 740 10480 17 0.55 0.4 air cooling after rolling 760 10 480 18 0.55 0.4 aircooling after rolling 760 10 480 19 0.55 0.4 air cooling after rolling760 10 480 20 0.44 0.3 7 820 730 11 450 21 0.59 0.4 15 820 800 12 450 220.59 0.4 15 820 800 12 450 23 0.59 0.4 15 820 800 12 450 24 0.59 0.4 15820 800 12 450 25 0.44 0.3 24 780 770 20 550 26 0.36 0.2 28 760 760 25480 Metallurgical property of head of the rails Metallurgy structure ofhead Hardness of Total elongation *3 Method No. of rails *1 rails *2(Hv)(%) Inventive 1 pearlite 390 15.0 2 pearlite 342 16.5 3 pearlite 40214.5 4 pearlite 425 14.5 5 pearlite 445 13.2 6 pearlite 430 12.5 7pearlite 440 11.9 8 pearlite 420 11.2 9 pearlite 455 12.4 10 pearlite430 11.7 11 pearlite 450 11.5 12 pearlite 450 10.5 13 pearlite 475 11.514 pearlite 440 10.1 15 pearlite 470 9.0 16 pearlite 445 14.0 17pearlite 450 12.7 18 pearlite 455 13.2 19 pearlite 460 12.9 20 pearlite470 13.8 21 pearlite 445 12.1 22 pearlite 440 12.5 23 pearlite 440 12.324 pearlite 430 12.7 25 pearlite 440 11.2 26 pearlite 470 9.9*1: structure 2 mm under the surface of the head of the rails*2: hardness 2 mm under the surface of the head of the rails*3: the elongation of the specimen 5 mm below the surface of the head ofrails at tensile test (See FIG. 6)

TABLE 3 Comparison method Carbon content and PC Value Hot rollingconditions PC Number of Range of cross- Maximum temperature C content(C, value rollings section reduction of the head of rails (T, (CPT1)Method No. Steel mass %) V + 10 × Nb + 5 × N P(times) (%) ° C.) 800/C. ×T Comparison 27 A 0.86 0.00 2  5˜20 1000 0.93 28 B 0.90 0.00 4 10˜30 950— 29 D 0.95 0.18 4 10˜24 950 — 30 E 1.00 0.00 5  8˜18 875 — 31 F 1.000.00 6  5˜15 900 — 32 G 1.10 0.00 4  4˜25 930 — 33 G 1.10 0.00 2 10˜28920 0.79 34 H 1.10 0.00 6  5˜10 850 — 35 I 1.20 0.00 5 10˜30 900 — 36 J1.40 0.00 5 10˜30 940 — 37 V 1.10 0.50 4  4˜25 930 — 38 G 1.10 0.00 4 4˜25 930 — 39 F 1.00 0.00 5 10˜20 1000 — 40 G 1.10 0.00 4  4˜25 930 —41 A 0.86 0.00 3  8˜24 950 — 42 G 1.10 0.00 2 10˜28 920 0.79 43 H 1.100.00 6  5˜10 850 — 44 D 0.95 0.00 4 10˜24 950 — Heat treatment Hotrolling conditions Cooling condition after rolling condition afterrolling Maximum interval Cooling Temperature Temperature of Cooling(CPT2) time of rolling (S, rate (° C./ of ending starting heat rate (°C./ Method No. 2400/C. × T × P sec.) sec.) cooling (° C.) treatment(°C.) sec.) Comparison 27 — 3.0 5 900 750 7 28 0.70 6.0 7 900 890 5 290.66 1.2 8 870 740 10 30 0.55 2.0 air cooling 760 10 31 0.44 1.0 7 820730 11 32 0.59 0.8 15 820 800 12 33 — 1.0 18 800 750 14 34 0.43 0.6 20780 780 15 35 0.44 0.9 24 780 770 20 36 0.36 0.4 28 760 760 25 37 0.590.4 15 820 800 12 38 0.55 0.4 2 (slow rate) 820 800 12 39 0.48 0.2 11960 (high temp.) 770 6 40 0.55 0.4 15 720 (low temp.→ high recup.) 70012 41 0.98 0.7 6 945 710 35 (high rate) 42 — 0.6 18 800 680 (low temp.)14 43 0.43 0.1 20 780 780 1 (low rate) 44 0.66 0.5 8 870 740 10 Heattreatment condition after rolling Temperature of Metallurgical propertyof head of the rails ending cooling Metallurgy structure of headHardness of Method No. (° C.) of rails *1 rails *2(Hv) Total elongation*3(%) Comparison 27 580 pearlite 390 13.6 28 510 pearlite 402 12.4 29480 pearlite 445 12.1 30 480 pearlite 440 10.0 31 450 pearlite 455 9.532 450 pearlite 430 9.7 33 480 pearlite 450 9.2 34 500 pearlite 475 9.035 550 pearlite 440 8.2 36 480 pearlite 470 7.5 37 450 pearlite 430 11.0(coarse deposition, small ductility improvement) 38 450 pearlite(coarse)430 8.8 (grain growth) 39 510 pearlite(coarse) 420 9.5 (grain growth) 40450 pearlite(coarse) 430 8.5 (grain growth) 41 520 pearlite + martensite560 5.0 (ductility deteriorated) 42 480pearlite + initial deposition of cementite 385 6.9 (ductilitydeteriorated) 43 500 pearlite + initial deposition of cementite 345 6.2(ductility deteriarated) 44 620 (low tempe.→high recup.) pearlite(coarse) 336 7.6 (ductility deteriorated)*1: structure 2 mm under the surface of the head of the rails*2: hardness 2 mm under the surface of the head of the rails*3: the elongation of the specimen 5 mm below the surface of the head ofrails at tensile test (See FIG. 6)temp. = temperaturerecup. = recuperation

1. A method for producing a steel rail having a high content of carbon,wherein the rail contains, in mass %, C: more than 0.85% but less thanor equal to 1.40%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, B: 0.0001 to0.0050%, optionally one or more selected from Cr: 0.05 to 2.00%, Mo:0.01 to 0.50%, Co: 0.003 to 2.00%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%,Ti: 0.0050 to 0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0150%, Al:0.0100 to 1.00%, Zr: 0.0001 to 0.2000%, N: 0.0060 to 0.0200%, V: 0.005to 0.500% and Nb: 0.002 to 0.050%, and the balance being Fe andunavoidable impurities, comprising: finish rolling said rail in twoconsecutive passes, with a reduction rate per pass of a cross-section ofsaid rail of 2-30%, wherein conditions of said finish rolling satisfythe following relationship:S≦CPT1 wherein CPT1 is the value expressed by the following expression 1CPT1=800/(C×T)  (expression 1) wherein S is the maximum rolling intervaltime (seconds), and (C×T) is defined as follows; C is the carbon contentof the steel in mass %, and T is the maximum surface temperature (° C.)of a rail head.
 2. A method for producing a steel rail having a highcontent of carbon in mass %, C: more than 0.85% but less than or equalto 1.40%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, B: 0.0001 to 0.0050%,optionally one or more selected from Cr: 0.05 to 2.00%, Mo: 0.01 to0.50%, Co: 0.003 to 2.00%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Ti:0.0050 to 0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0150%, Al:0.0100 to 1.00%, Zr: 0.0001 to 0.2000%, N: 0.0060 to 0.0200%, V: 0.005to 0.500% and Nb: 0.002 to 0.050%, and the balance being Fe andunavoidable impurities, comprising: finish rolling said rail in three ormore passes, with a reduction rate per pass of a cross-section of saidrail of 2-30%, wherein conditions of said finish rolling satisfy thefollowing relationship:S≦CPT2 wherein CPT2 is the value expressed by the following expression2,CPT2=2400/(C×T×P)  (expression 2) wherein S is the maximum rollinginterval time (seconds), and (C×T×P) is defined as follows; C is thecarbon content of the steel rail in mass %, and T is the maximum surfacetemperature (° C.) of a rail head, and P is the number of passes, whichis 3 or more. 3-12. (canceled)
 13. The method according to claim 1,wherein chemical composition(s) included in said rail meet the followingrelationship:0.30≧V (mass %)+10×Nb (mass %)+5×N (mass %)≧0.04
 14. The methodaccording to claim 1, further comprising: immediately after said finishrolling, cooling the surface of said rail head at a cooling rate of2-30° C./sec. until the surface temperature reaches 950-750° C.
 15. Themethod according to claim 14, further comprising: after said coolingstep, when the temperature of the rail head is more than 700° C.,cooling the surface of the rail head at a cooling rate of 2-30° C./sec.until the surface temperature reaches at least 600° C.; and thenallowing the rail to further cool at room temperature.
 16. The methodaccording to claim 1, further comprising: after said finish rollingprocess, when the temperature of the rail head is more than 700° C.,cooling the surface of the rail head at a cooling rate of 2-30° C./sec.until the surface temperature reaches at least 600° C., and thenallowing the rail to further cool at room temperature.
 17. The methodaccording to claim 2, wherein chemical composition(s) included in saidrail meet the following relationship:0.30≧V (mass %)+10×Nb (mass %)+5×N (mass %)≧0.04
 18. The methodaccording to claim 2, further comprising: immediately after said finishrolling, cooling the surface of said rail head at a cooling rate of2-30° C./sec. until the surface temperature reaches 950-750° C.
 19. Themethod according to claim 18, further comprising: after said coolingstep, when the temperature of the rail head is more than 700° C.,cooling the surface of the rail head at a cooling rate of 2-30° C./sec.until the surface temperature reaches at least 600° C.; and thenallowing the rail to further cool at room temperature.
 20. The methodaccording to claim 2, further comprising: after said finish rollingprocess, when the temperature of the rail head is more than 700° C.,cooling the surface of the rail head at a cooling rate of 2-30° C./sec.until the surface temperature reaches at least 600° C., and thenallowing the rail to further cool at room temperature.